Gas turbine
A
gas turbine, also called a
combustion turbine, is a rotary
engine that extracts energy from a flow of
combustion gas. It has an upstream
compressor coupled to a downstream
turbine, and a combustion chamber in-between. (
Gas turbine may also refer to just the
turbine element.)
Energy is released when
air is mixed with
fuel and
ignited in the
combustor. A common mistake is that combustion increases the pressure of the gasses flowing through a turbine. In fact the heat addition stage of a gas turbine cycle incurs a slight pressure drop to facilitate flow through the engine. For all intents and purposes, however, the combustion process can be considered as occuring at constant pressure, with an increasing
volume to accommodate the temperature rise, as explained by the
ideal gas law. This in turn results in an increase in the
velocity of the gas flow (see
gas laws). This is directed over the turbine's blades, spinning the turbine and powering the compressor, and finally is passed through a
nozzle, generating additional thrust by accelerating the hot exhaust gases by expansion back to atmospheric pressure.
Energy is extracted in the form of shaft power, compressed air and thrust, in any combination, and used to power
aircraft,
trains,
ships,
generators, and even
tanks.
1500: The 'Chimney Jack' was drawn by
Leonardo da Vinci which was turning a roasting spit. Hot air from a fire rose through a series of fans which connect and turn the roasting spit.
1629: Jets of steam rotated a turbine that then rotated driven machinery allowed a stamping mill to be developed by
Giovanni Branca.
1678: Ferdinand Verbeist built a model carriage relying on a steam jet for power.
1791: A basic turbine engine was patented with all the same elements as today's modern gas turbines. The turbine was designed to power a horseless carriage.
1872: The first true gas turbine engine was designed by Dr F. Stolze, but the engine never ran under its own power.
1897: A
steam turbine for propelling a ship was patented by Sir Charles Parson. This principle of propulsion is still of some use.
1903: A Norwegian,
Ægidius Elling, was able to build the first gas turbine that was able to produce more power than needed to run its own components, which was considered an achievement in a time when knowledge about aerodynamics was limited. Using rotary compressors and turbines it produced 11 hp(massive for those days). His work was later used by Sir
Frank Whittle.
1914: The first application for a gas turbine engine was filed by Charles Curtis.
1918: One of the leading gas turbine manufacturers of today,
General Electric, started their gas turbine division.
1920. The then current gas flow through passages was developed by Dr A. A. Griffith to a turbine theory with gas flow past airfoils.
1930. Sir
Frank Whittle patented the design for a gas turbine for
jet propulsion. His work on gas propulsion relied on the work from all those who had previously worked in the same field and he has himself stated that his invention would be hard to achieve without the works of Ægidius Elling. The first successful use of his engine was in April 1937.
1936. Hans von Ohain and Max Hahn in Germany developed their own patented engine design at the same time that Sir
Frank Whittle was developing his design in England.
Gas turbines are described
thermodynamically by the
Brayton cycle, in which air is compressed
isentropically,
combustion occurs at constant pressure, and expansion over the turbine occurs isentropically back to the starting pressure.
In practice, friction and turbulence cause:
a) non-isentropic compression - for a given overall pressure ratio, the compressor delivery temperature is higher than ideal.
b) non-isentropic expansion - although the turbine temperature drop necessary to drive the compressor is unaffected, the associated pressure ratio is greater, which decreases the expansion available to provide useful work.
c) pressure losses in the air intake, combustor and exhaust - reduces the expansion available to provide useful work.
As with all cyclic
heat engines, higher combustion temperature means greater
efficiency. The limiting factor is the ability of the steel, ceramic, or other materials that make up the engine to withstand heat and pressure. Considerable engineering goes into keeping the turbine parts cool. Most turbines also try to recover exhaust heat, which otherwise is wasted energy.
Recuperators are
heat exchangers that pass exhaust heat to the compressed air, prior to combustion.
Combined cycle designs pass waste heat to
steam turbine systems. And
combined heat and power (co-generation) uses waste heat for hot water production.
Mechanically, gas turbines can be considerably less complex than
internal combustion piston engines. Simple turbines might have one moving part: the shaft/compressor/turbine/alternator-rotor assembly (see image above), not counting the fuel system.
More sophisticated turbines (such as those found in modern
jet engines) may have multiple shafts (spools), hundreds of turbine blades, movable stator blades, and a vast system of complex piping, combustors and heat exchangers.
As a general rule, the smaller the engine the faster the shaft(s) rotate to maintain tip speed;
jet engines operate around 10,000 rpm and micro turbines around 100,000 rpm.
Thrust bearings and
journal bearings are a critical part of design. Traditionally, they have been
hydrodynamic oil bearings, or oil-cooled
ball bearings. This is giving way to hydrodynamic
foil bearings, which have become common place in micro turbines and APU's (auxiliary power units.)
See
jet engine page.
Industrial gas turbines range in size from truck-mounted mobile plants to enormous, complex systems.
The power turbines in the largest industrial gas turbines operate at 3,000 or 3,600
rpm to match the
AC power grid frequency and to avoid the need for a reduction gearbox. Such engines require a dedicated building.
They can be particularly efficient â€" up to 60
% â€" when waste heat from the gas turbine is recovered by a conventional steam turbine in a
combined cycle configuration. They can also be run in a
cogeneration configuration: the exhaust is used for space or water heating, or drives an
absorption chiller for cooling or refrigeration.
Simple cycle gas turbines in the power industry require smaller capital investment than combined cycle gas,
coal or
nuclear plants and can be designed to generate small or large amounts of power. Also, the actual construction process can take as little as several weeks to a few months, compared to years for baseload plants. Their other main advantage is the ability to be turned on and off within minutes, supplying power during peak demand. Large simple cycle gas turbines may produce several hundred megawatts of power and approach 40 %
thermal efficiency.
Also known as:
*Minature Gas Turbines
*Micro-jets
Many model engineers relish the challenge of re-creating the grand engineering feats of today as tiny working models. Naturally, the idea of re-creating a powerful engine such as the jet, fascinated hobbyists since the very first full size engines were powered up by
Hans von Ohain and
Frank Whittle back in the 1930s.
Re-creating machines such as engines to a different scale is not easy. The laws of physics governing the behaviour of many machines do not always scale up or down at the same rate as the machine's size (and often not even in a linear way), usually at best causing a dramatic loss of power or efficiency, and at worst causing them not to work at all. An
automobile engine, for example, will not work if re-produced in the same shape as the size of a human hand.
With this in mind the pioneer of modern Micro-Jets,
Kurt Schreckling, produced one of the world's first Micro-Turbines, the FD3/67. This amazing little engine can give out 22
newtons of thrust, and can be built by most mechanically minded people with basic engineering tools, such as a
metal lathe.
Also known as:
*Turbo alternators
*Gensets
*MicroTurbine® (registered trademark of
Capstone Turbine Corporation)
*Turbogenerator® (registered tradename of
Honeywell Power Systems, Inc.)
Micro turbines are becoming wide spread for
distributed power and
combined heat and power applications. They range from handheld units producing less than a
kilowatt to commercial sized systems that produce tens or hundreds of kilowatts.
Part of their success is due to advances in electronics, which allow unattended operation and interfacing with the commercial power grid. Electronic power switching technology eliminates the need for the generator to be synchronized with the power grid. This allows, for example, the generator to be integrated with the turbine shaft, and to double as the starter motor.
Micro turbine systems have many advantages over piston engine generators, such as higher power density (with respect to footprint and weight), extremely low emissions and few, or just one, moving part. Those designed with
foil bearings and air-cooling operate without oil, coolants or other hazardous materials. However, piston engine generators are quicker to respond to changes in output power requirement.
They accept most commercial fuels, such as
natural gas,
propane,
diesel and
kerosene. The are also able to produce
renewable energy when fueled with
biogas from
landfills and
sewage treatment plants.
Micro turbine designs usually consist of a single stage radial compressor, a single stage
radial turbine and a recuperator. Recuperators are difficult to design and manufacture because they operate under high pressure and temperature differentials. Exhaust heat can be used for water heating, drying processes or absorption chillers, which create cold for air conditioning from heat energy instead of electric energy.
Typical micro turbine efficiencies are 25 to 35 %. When in a combined heat and power
cogeneration system, efficiencies of greater than 80 % are commonly achieved.
Auxiliary power units (APUs) are small gas turbines designed for auxiliary power of larger machines, usually
aircraft. They are well suited for supplying compressed air for aircraft ventilation (with an appropriate compressor design), start-up power for larger
jet engines, and electrical and hydraulic power. (These are not to be confused with the auxiliary propulsion units, also abbreviated APUs, aboard the gas-turbine-powered Oliver Hazard Perry-class guided-missile frigates. The Perrys' APUs are large electric motors that provide maneuvering help in close waters, or emergency backup if the gas turbines are not working.)
Gas turbines are used on
ships,
locomotives,
helicopters, and in
tanks. A number of experiments have been conducted with gas turbine powered
automobiles.
In 1950, designer F. R. Bell and Chief Engineer Maurice Wilks from British car manufacturers
Rover unveiled the first car powered with a gas turbine engine. The two-seater JET1 had the engine positioned behind the seats, air intake grilles on either side of the car and exhaust outlets on the top of the tail. During tests, the car reached top speeds of 140 km/h, at a turbine speed of 50,000 rpm. The car ran on
petrol,
paraffin or
diesel oil, but fuel consumption problems proved insurmountable for a production car. It is currently on display at the London
Science Museum. Rover and the BRM
Formula One team joined forces to produce a gas turbine powered coupe, which entered the 1963
24 hours of Le Mans, driven by
Graham Hill and Richie Ginther. It averaged 107.8 mph (173 km) and had a top speed of 142 mph (229 km/h). In 1971 Lotus principal Colin Chapman introduced the Lotus 56B F1 car, powered by a Pratt & Whitney gas turbine. Colin Chapman had a reputation of building radical championship-winning cars, but had to abandon the project because there were too many problems with turbo lag. The fictional
Batmobile is often said to be powered by a gas turbine or a
jet engine.
American car manufacturer
Chrysler demonstrated several
prototype gas turbine-powered cars from the early 1950s through the early 1980s. Chrysler built fifty
Chrysler Turbine Cars in 1963 and conducted the only consumer trial of gas turbine-powered cars.
In
1993 General Motors introduced the first commercial gas turbine powered
hybrid vehicleâ€"as a limited production run of the
EV-1. A
Williams International 40 kW turbine drove an alternator which powered the battery-electric powertrain. The turbine design included a
recuperator.
Gas turbines offer a high-powered engine in a very small and light package. However, they are not as responsive and efficient as small piston engines over the wide range of RPMs and powers needed in vehicle applications. Also, turbines have historically been more expensive to produce than piston engines, though this is partly because piston engines have been mass-produced in huge quantities for decades, while small turbines are rarities. It is also worth noting that a key advantage of jets and
turboprops for aeroplane propulsion - their superior performance at high altitude compared to piston engines, particularly
naturally-aspirated ones - is irrelevant in automobile applications. Their power-to-weight advantage is far less important. Their use in hybrids reduces the responsiveness problem. Capstone currently lists on their website a version of their turbines designed for installation in hybrid vehicles.
The
MTT Turbine SUPERBIKE appeared in 2000 (hence the designation of Y2K Superbike by MTT) and is the first production motorcycle powered by a jet engine - specifically, a Rolls-Royce Allison model 250 turboshaft engine, producing about 283kW (380shp). Speed-tested to 365km/h or 227mph (according to some stories, the testing team ran out of road during the test), it holds the Guinness World Records for most powerful production motorcycle and most expensive production motorcycle, with a price tag of US$185,000.
Use of gas turbines in military tanks has been more successful. In the 1950s, a
Conqueror heavy tank was experimentally fitted with a
Parsons 650-hp gas turbine, and they have been used as
auxiliary power units in several other production models. Today, the Soviet/Russian
T-80 and U.S.
M1 Abrams tanks use gas turbine engines. See
tank for more details.
Several locomotive classes have been powered by gas turbines, the most recent incarnation being
Bombardier's
JetTrain. See
Gas turbine-electric locomotive for more information.
Naval use
Gas turbines are used in many naval vessels, where they are valued for their high power-to-weight ratio and their ships' resulting acceleration and ability to get underway quickly. The first gas-turbine-powered naval vessel was the
Royal Navy's Motor Gun Boat
MGB 2009 (formerly
MGB 509) converted in 1947. The first large, gas-turbine powered ships, were the Royal Navy's
Type 81 (Tribal class) frigates, the first of which (
HMS Ashanti) was commissioned in
1961.
The next series of major naval vessels were the four
Canadian Iroquois class helicopter carrying destroyers first commissioned in 1972. They used 2 FT-4 main propulsion engines, 2 FT-12 cruise engines and 3 Solar Saturn 750 KW generators.
The first U.S. gas-turbine powered ships were the
U.S. Coast Guard's
Hamilton-class High Endurance Cutters the first of which (
USCGC Hamilton) commissioned in
1967. Since then, they have powered the
U.S. Navy's
Perry-class frigates,
Spruance-class and
Arleigh Burke-class destroyers, and
Ticonderoga-class guided missile cruisers.
USS Makin Island, a modified
Wasp-class amphibious assault ship, is to be the Navy's first
amphib powered by gas turbines.
Three
Rolls-Royce gas turbines power the
118 WallyPower, a 118 foot super-yacht. These engines combine for a total of 16,800HP allowing this 118 foot boat to maintain speeds of 60
knots or 70
mph. Another example of commercial usage of a gas turbine in a ship is the Stena Discovery, using the GE LM2500.
A popular hobby is to construct a gas turbine from an automotive
turbocharger. A combustion chamber is fabricated and plumbed between the compressor and turbine. Like many technology based hobbies, they tend to give rise to manufacturing businesses over time. Several small companies manufacture small turbines and parts for the amateur. See external links for resources.
Gas turbine technology has steadily advanced since its inception and continues to evolve; research is active in producing ever smaller gas turbines. Computer design, specifically
CFD and
finite element analysis along with material advances, has allowed higher compression ratios and temperatures, more efficient combustion, better cooling of engine parts and reduced emissions. Additionally, compliant
foil bearings were commercially introduced to gas turbines in the
1990s. They can withstand over a hundred thousand start/stop cycles and eliminated the need for an oil system.
On another front, microelectronics and power switching technology have enabled commercially viable micro turbines for distributed and vehicle power. An excellent example is the Capstone line of micro turbines, which do not require an oil system and can run unattended for months on end!
*
Turbine*
Jet engine*
Brayton cycle*
Gas Turbine Engines for Model Aircraft by
Kurt Schreckling, ISBN 0 9510589 1 6 Traplet Publications
* "Aircraft Gas Turbine Technology" by Irwin E. Treager, Professor Emeritus Purdue University, McGraw-Hill, Glencoe Division, 1979, ISBN 0-02-801828
*
Model Turbine Engine*
MIT Gas Turbine Laboratory*
MIT Microturbine research*
First Marine Gas Turbine 1947*
A history of Chrysler turbine cars*
DIY Gas Turbines Yahoo Group
